This invention relates to molten metal coatings and particularly to the attainment of a so-called minimized spangle surface upon a hot dip galvanized coating.
As discussed in U.S. Pat. Nos. 3,608,520 and 3,717,501 listed above there have been a number of processes suggested for treatment of molten zinc surfaces on sheet and strip passing from a molten zinc bath in order to accellerate crystallization of the surface of the coating and obtain very small crystals or spangles on the surface of such coating. Molten coating surfaces on such sheets have in the past been contacted with water sprays, steam blasts, zinc powder or other crystallization nucleating substances blown onto the surface, and cooled rolls contacting the surface just prior to solidification in order to enhance nucleation. All of these expedients have had certain drawbacks. The internally cooled roll arrangement has the particular advantage of being cleaner and less troublesome than other nucleating processes. For example, water sprays or steam jets applied to the coating cause extremely humid conditions adjacent such sprays or jets often resulting in severe corrosion of the line apparatus.
The inventions of U.S. Pat. Nos. 3,608,520 and 3,717,501 were improvements of a previous invention disclosed in U.S. Pat. No. 1,933,401 to Ward wherein an internally cooled roll was used to enhance nucleation and smooth the coating while obtaining a minimized spangle without the disadvantages of other methods of nucleation. The '401 patent used a point contact of the cooling roll with the molten coated surface, while the '501 and '520 patents used an improved partial wraparound arrangement of the strip or sheet upon unequal diameter offset cooled rolls. Although the surface of the minimized spangle rolls of the '520 and '501 patents was made very smooth, however, and the sheet and strip was wrapped tightly about such rolls in order to obtain good contact and heat conduction for a suitable period to obtain rapid cooling of the molten zinc surface, it has been found that the minimized spangle obtained is frequently not suppressed completely or is nonuniform. Furthermore, in some cases, particularly in the vicinity of scratches, welts, dross particles on the surface of the coating, or dents in the base metal, the usual spangle appears in the area about such blemishes.